Semi-crystalline macromolecules are globally metastable, multi-phase systems with phase dimensions ranging from micrometers to nanometers. The polymer molecules, being usually longer than one micrometer, cross the boundaries and decouple at the interfaces. This decoupling is often not complete and different degrees of influence are extended across the interfaces. Thermodynamically, crystals can be characterized by their melting behavior and non-crystalline phases by their glass transition. On weak coupling, the non-crystalline segments only show a broadening of the glass transition to higher temperature. With stronger coupling, non-crystalline material may remain solid at the transition of the bulk-amorphous phase and form a separate, rigid-amorphous nanophase, or rigid amorphous fraction, RAF. The RAF undergoes its glass transition either below, at, or even above, the melting temperature. In the presence of a RAF, the semi-crystalline polymers may be a system of three or more types of phases with different relaxation effects due to the coupling between the phases. This and other examples of decoupling are discussed here and a general concept is developed. This applies to positional decoupling at positions of chemical changes within the molecule, such as in copolymers, and to physical changes, such as in entanglements, and is not limited to decoupling at interfaces. Finally, it is pointed out that there is also the possibility of a temporal decoupling of thermodynamically simultaneous changes, which on sufficiently slow kinetics in one may change to consecutive changes. Many of these aspects of decoupling on a molecular scale influence the macroscopic properties and must be considered for the analysis and application of modern materials.